Integrated optical waveguides are well known in the art, and typically comprise a patterned, light guiding core layer (of refractive index n1) surrounded by a cladding material (of refractive index n2, where n2<n1) and mounted on a suitable substrate. Light propagating along the waveguide is guided within the core by the refractive index difference between core and cladding.
FIGS. 1a and 1b show side and end views of a typical integrated optical waveguide 10, comprising a substrate 11, a lower cladding layer 12, a light guiding core 13 and an upper cladding layer 14. Depending on the material system, a variety of techniques are available for depositing the lower cladding, core and upper cladding layers. These include flame hydrolysis, chemical vapour deposition and physical vapour deposition (eg. for glass), molecular beam epitaxy (eg. for semiconductors) and spin coating, spray coating and roller coating (eg. for polymers). A core layer may be patterned for example by photolithography and reactive ion etching (suitable for most materials) or by photolithography and wet etching (eg. for photo-patternable polymers), exploiting a solubility differential between exposed and unexposed material.
Irrespective of the method used to fabricate an integrated optical waveguide, the refractive index of the lower 12 and upper 14 cladding layers needs to be less than that of the core 13, so that light is confined within the core. Often, the lower 12 and upper 14 cladding layers have the same refractive index, so that the core-guided mode is symmetric, although this is not essential. If the substrate material is transparent and has refractive index lower than the core material, the lower cladding 12 may be omitted. Typically, waveguides have a light transmissive elongated core region that is square or rectangular in cross section, as illustrated in FIG. 1. The bottom face is conventionally defined as that being adjacent or nearest the substrate, the top face as the face parallel to the bottom face but furthest from the substrate, and the sides as those faces that are perpendicular to the substrate.
Much of the early developmental work on integrated optical waveguides, notably at NTT (M. Kawachi, “Recent progress in silica-based planar lightwave circuits on silicon”, IEE Proc Optoelectronics vol. 143(5), pp 257-262, 1996) and AT&T Bell Labs (Y. P. Li and C. H. Henry, “Silica-based optical integrated circuits”, IEE Proc Optoelectronics vol. 143(5), pp. 263-280, 1996) was aimed at producing devices for optical telecommunications networks, operating mostly at 1.55 μm. For compatibility with the silicate optical fibres that formed the network backbone, the integrated optical waveguides were generally likewise composed of silicate glass, often on silicon substrates. This material system is often known in the art as “silica on silicon”, and has been applied to a number of telecommunications devices including 1×2 switches, 1×N splitters, M×N star couplets and arrayed waveguide gratings (AWGs). A disadvantage with silica-based waveguides is the large capital cost of the fabrication process, requiring for example flame hydrolysis or chemical vapour deposition for depositing the various layers, and photoresist patterning followed by reactive ion etching for patterning the core layer.
Alternative waveguide material systems have been explored for telecommunications devices, including silicon-on-insulator and highly fluorinated polymers. Optical polymers are a particularly favourable material system if they are photo-patternable, because the capital cost of the fabrication plant is considerably less than required for silicate (or silicon) waveguides. Several methods for the fabrication of optical waveguides from photo-patternable polymers are known in the art. One such method, which may be described as a ‘UV lithography/wet etch’ method and disclosed for example in U.S. Pat. No. 4,609,252, U.S. Pat. No. 6,054,253 and U.S. Pat. No. 6,555,288, involves deposition of a layer of photo-curable liquid polymer or polymer solution onto a substrate, followed by image-wise exposure of the photo-curable polymer to actinic radiation, usually ultraviolet (UV) light. The patterned polymer layer is then flushed with a developing solvent, exploiting a solubility differential between exposed and unexposed material. With photo-curable polymers, the exposed material is generally less soluble than the unexposed material, and is left behind by the solvent, similar to well known “negative tone” photoresist. Alternatively, the exposure may cause the material to be more soluble, in which case the unexposed material is left behind (similar to “positive tone” photoresist). Apart from UV light, several types of actinic radiation suitable for curing photo-curable polymers are known in the art, including X-rays, visible light and electron beams. Several techniques for depositing the polymer material are known in the art, with spin coating generally considered to be the method of choice for depositing optical quality polymer layers. The image-wise exposure can be performed with light either through a mask, eg. in a mask aligner or stepper, or by a laser direct writing procedure; exposure through a mask is generally preferred for high fabrication throughput. A general “negative tone” procedure for fabricating an optical waveguide from UV-patternable polymers is illustrated in FIGS. 2a to 2d. As shown in FIG. 2a, a low refractive index UV-curable polymer is spin coated onto substrate 20 and blanket exposed to UV light to form a lower cladding layer 21. As shown in FIG. 2b, a high refractive index UV-curable polymer is spin coated onto lower cladding layer 21, then image-wise exposed to UV light 22 through a mask 23 to produce a region of UV-exposed material 24 and a region of unexposed material 25. FIG. 2c shows a core 26 comprised of UV-exposed material 24, after the unexposed material 25 has been removed with a solvent, in a step commonly known as “wet development” or “wet etching”. Finally, FIG. 2d shows an upper cladding layer 27 formed by spin coating and blanket UV exposure of another low refractive index UV-curable polymer.
In a variation on this process that may be applicable for polymers deposited from solution, disclosed for example in U.S. Pat. No. 3,689,264 and U.S. Pat. No. 3,809,732, the imagewise UV exposure produces the necessary refractive index change (positive or negative) to differentiate the core region from the cladding region, so that a wet development stage is not required. Alternatively, a polymer core layer may be patterned by a moulding or embossing process, as disclosed in U.S. Pat. No. 5,230,990 and U.S. Pat. No. 5,265,184 for example, where waveguides can be produced repeatedly once a master or mould has been obtained. Photo-curable polymers are particularly suitable in this case, because they can be cured (ie. solidified) during the moulding or embossing process. In yet another alternative, a curable polymer core layer may be deposited as required by a direct dispense and cure process, as disclosed in U.S. Pat. No. 5,534,101 for example. Other methods for fabricating integrated optical waveguides from polymer materials will be known to those skilled in the art.
In another variation, the polymer materials may be thermally curable rather than photo-curable. A thermally curable polymer layer may be blanket cured by heat, or patterned in an analogous manner to a photo-curable polymer using a spatially selective source of heat such as an infrared laser, followed by wet development if required. In general, photo-curable polymers are preferred to thermally curable polymers, because the short wavelength of the curing light (usually UV) enables superior spatial precision in the patterning process. Photo-curable polymers also tend to have superior shelf life.
The term polymer as used herein refers to a substantially organic molecule of high relative molecular mass, the structure of which comprises portions having multiple repetitions of units derived actually or conceptually from molecules of low relative molecular mass. Most polymers are composed of repeating carbon-based units, although siloxanes (also known as silicones), composed of a repeating Si—O backbone or network with carbon-based groups attached to the silicon atoms, are also considered to be polymers. For the purposes of this specification, a curable polymer comprises molecules (monomers, oligomers or macromolecules) capable of entering, through reactive groups, into polymerisation or further polymerisation, thereby contributing more than one monomeric unit to the final polymer.
Integrated optical waveguides have many potential applications besides telecommunications devices. One such application is optical backplanes for high speed computers. Another is in waveguide-based optical touch screen sensors described in U.S. Pat. No. 5,914,709, U.S. Pat. No. 6,181,842 and U.S. Pat. No. 6,351,260, which may be employed in a variety of consumer electronics devices including hand-held games, mobile phones, computers and personal digital assistants (PDAs). The potential markets for such devices are huge, and if optical waveguides are to be used in them, it is essential that methods be found to mass-produce them in a cost-effective manner.
It will be appreciated by those skilled in the art that photo-patternable polymers ale attractive as low cost waveguide materials. Firstly, as explained above, the capital cost of the fabrication plant is low. Secondly, there are cost reductions associated with higher throughput, since most polymer waveguide fabrication methods, such as the photolithography/wet etch procedure shown in FIGS. 2a to 2d, take considerably less time than procedures for fabricating silica-on-silicon waveguides (chemical vapour or flame hydrolysis deposition of the silica material; photoresist deposition, patterning and development; and reactive ion etching of the silica material). In a further cost advantage, polymers are relatively inexpensive waveguide materials. It is important to note that if there is no necessity to operate in the 1.55 μm telecommunications window, there is no need for expensive highly fluorinated polymers (eg. as disclosed in U.S. Pat. No. 6,308,001 and U.S. Pat. No. 6,555,288) that have been developed for low optical loss telecommunications devices. Instead, the optics in consumer electronics devices can be designed to operate at wavelengths where standard hydrocarbon polymers have minimal optical absorption, in particular in the near IR region below 1 μm.
For the purposes of this specification, the thickness uniformity of a layer is defined in a relative manner, as (standard deviation in thickness/average thickness)*100%.
Although photo-patternable polymers are attractive as low cost waveguide materials, there remains a need for reliable and reproducible methods for scaling up the fabrication of optical waveguides for high volume applications such as optical backplanes and consumer electronics devices.